This invention relates to hybrid laminates built up from two or more alternating layers of non-ferrous alloy sheet and fibre layers which are impregnated with resin, bonded together during a time/temperature cycle under pressure. The metal sheets are on both the outer faces.
A version of such a laminate is ARALL which is typically built-up from aluminium alloy sheets thinner than one millimeter and intermediate layers of impregnated aramid fibres. The built-up of the ARALL laminate can be coded according to the ALCOA ARALL material specification. A commercially introduced 5/4 ARALL laminate by Alcoa for example, consists of five sheets of an aluminium alloy, each sheet having a thickness of 0.3 mm, and four intermediate layers of unidirectional fibre/resin, each having a thickness of 0.2 mm. A digit added to the code indicates the quality of the metal sheet used. For example, ARALL 3 refers to sheets of aluminium alloy 7475-T761 according to Aerospace Material Specification SAE AMS 4085B. The code T761 indicates the heat treatment of the sheet used which is a solution heat treatment, quenching, double stage precipitation heat treatment and stabilisation. By using metal sheets of different alloys and tempers, or different adhesive/fibre layers (fibre type and resin type), or different fibre orientations, or by applying more or less layers, a wide range of hybrid laminates with different characteristics can be obtained.
Hybrid laminates are used instead of monolithic material because of the improved characteristics with regard to strength, damage tolerance, lower density and good damping characteristics. Referring to the durability properties, the fatigue strength is considerably better than that of monolithic material. The sigma 0.2, the strengt and fatigue resistance of hybrid metal laminates in general are substantially higher than of monolithic AL alloys. Moreover, the specific weight is between 8-15% lower. In the case of ARALL the elongation at failure in fibre direction is relatively low because of the small elongation of the aramide fibres. Forming of the laminate perpendicular to the fibres is limited due to the precipitated metal used.
The hybrid laminates can be used in the form of sheets, doublers and stiffeners. Aircraft structures for example comprise a combination of all three.